Although the present invention has general applicability to any given system where the formation and deposition of scale and in particular calcium scale is a potential problem, the invention will be discussed in detail as it concerns circulating cooling water systems. The present invention relates to methods for inhibiting scale deposits and fouling in circulating aqueous systems.
In industrial cooling systems, water such as from rivers, lakes, ponds, etc., is employed as the cooling media for heat exchangers. Such natural waters contain large amounts of suspended material such as silt, clay and organic wastes. The cooling water circulating through a heat exchanger is typically circulated through a cooling tower, spray pond or evaporative system prior to discharge or reuse. In such systems, cooling is achieved by evaporating a portion of the water passing through the system. Because of the evaporation which takes place during cooling, suspended materials in the water become concentrated. Fouling materials from the feedwater or as a result of the evaporative concentration can settle in locations of low flow rates and cause corrosion and inefficient heat transfer. Agglomerating agents such as polyacrylamides and polyacrylates have been used to agglomerate fine particles of mud and silt into a loose floc for removal. However, these flocs tend to settle in cooling tower basins and frequent cleaning is necessary to remove the settled flocs from the tower basins.
The water employed in industrial cooling water systems also often contains dissolved salts of calcium, magnesium, etc., which can lead to scale and sludge deposits. One of the most common scale deposits in circulating aqueous cooling systems is calcium carbonate. It normally results from the breakdown of calcium bicarbonate, a naturally occurring soluble salt. Calcium carbonate has a relatively low solubility and its solubility decreases with increasing temperature and pH. Thus, the rate of calcium carbonate deposition increases with increasing pH and temperature.
Deposit control agents such as phosphates, phosphonates and polyacrylates are often used to inhibit calcium carbonate scale formation in industrial cooling water systems. The use of polyacrylates alone is not effective at high calcium concentrations because undesirable polyacrylate-calcium adducts are formed reducing efficacy. Although phosphonates are very efficient at controlling calcium carbonate scale formation, they can produce insoluble phosphonate-calcium complexes or calcium phosphate scale upon degradation. Further, current limits on phosphate discharge limit the acceptability of the use of phosphonates for water treatments.
Preventing the corrosion and scaling of industrial heat transfer equipment is essential to the efficient and economical operation of a cooling water system. Excessive corrosion of metallic surfaces can cause the premature failure of process equipment requiring down time for the replacement or repair of the equipment. Additionally, the buildup of corrosion products on heat transfer surfaces impedes water flow and reduces heat transfer efficiency thereby limiting production or requiring down time for cleaning. Reductions in efficiency will also result from scaling deposits which retard heat transfer and hinder water flow.
Scale can also cause rapid localized corrosion and subsequent penetration of metallic surfaces through the formation of a differential oxygen concentration cells. The localized corrosion resulting from differential oxygen cells originating from deposits is commonly referred to as "under deposit corrosion". The treatment of industrial waters to inhibit scale formation with polyepoxysuccinic acid (hereinafter PESA) is disclosed in U.S. Pat. No. 5,062,962 incorporated herein by reference. The general formula for PESA is: ##STR1## wherein n ranges from about 2 to 50, preferably, 2 to 25, M is hydrogen or a water soluble cation such as Na.sup.+, NH.sub.4.sup.+ or K.sup.+ and R is hydrogen, C1-C4 alkyl or C1-C4 substituted alkyl (preferably R is hydrogen). PESA is known to be an effective inhibitor for scale control. However, it was found that when the PESA was employed in some systems that contained a variety of materials, both metal and plastic, and where the water was circulating, there was a slight decrease in efficacy of the scale inhibiting and deposit control properties of PESA.